The present invention relates to the steering of tracked machines such as crawler tractors which are steered by changing the speed of the track onone side of the machine relative to the track on the other side of the machine.
It is known to provide a machine with a hydrostatic transmission and differential steering where forward and reverse movement of the machine is caused by a primary drive input into opposed planetary differential gears sets and steering is achieved by a secondary drive input into the planetary differential gear sets. The differential gear sets combine the rotations of the primary and secondary inputs during steering for increasing the speed of one track on the machine while reducing the speed of the other track by an equal amount. The size of the hydraulic motors used in a steering system of this type is determined by the minimum turn radius required at a given speed. This normally calls for the use of large motors in order to meet the minimum turn radius requirement. These motors typically run below their rated capacity during periods of machine operation from the minimum turn radius up to straight running with consequential lower efficiency.
Thus, there has been a need for a steering system for tracked machines which is efficient at large turning radii while being capable of providing a small radius of turn as required. It is, therefore, an object of the present invention to provide both normal differential steering and a second method of steering where the speed of one machine track is reduced while keeping the other track at its initial speed for larger turning radii.